System for the recording of sound



June 2, 1942; l. R. GQSHAW 2,284,731

SYSTEM FOR THE nmconbme 0F scum) V Filed May 25,. 1940 q LIGHTTRANSM/Ssmu &

Patented June 2, 1942 2,284,731 sYs'rnM ron 'rnn nnooimmo OF SOUND IrlR. GoshamBeverly Bills, Calif, assignor to Radio Corporation of America,a corporation,

, of Delaware Application May 25, 1940, Serial No. 337,191

16 Claims.

This invention relates to sound recording systems, and particularly tothe recording or sound film records consisting of a plurality ofmutually related sound traces. Many types of sound film recordingsystems are known in the art, such as variable density, variable area,unilateral, bilateral, push-pull class A and push-pull class B, etc.

slit longitudinally disposed with respect to one another so that theapices of the beams 'just touch the slit or the center 01 the slit. Inthe 1 use of class B systems, an elongated extension is 1 the time thatthe other beam is effective.

The present invention is directed to a sound recording system forproducing a record combining the characteristics of the records producedby certain of these types, such as push-pull class A with class B,wherein the advantages of each type of system are obtained and thedisadvantages thereof eliminated.

Since the invention is directly related to the combination of push-pullclass A with class B, it

might be well to state that the push-pull class 1 A system of recordingon film consists in producing two mutually related tracks, eitherunilateral or bilateral, or variable density in which the modulations ineach trace are 180 out of phase with respect to one another, each trace,however, containing the complete modulations of the signal. The usualmethod oi. obtaining a push-pull class A track is to project two lightbeams upon a slit passing light to the film from each beam, and as thelight is decreased in one cut in the aperture mask producing the beamsto provide a narrow light beam on the film during realized that theconstruction of the class B recording system necessitates an accurateazimuth adjustment of the slitwith respect to the apices of thelightbeams inorder to insure that both positive and-negative cyclesoithe sound waves are properly impressed upon the film. Also, because ofthe substantial elimination of one of the .beamsduring the' negative andpositive cycles or the signal, greater care must be taken in theexposure and processing of such records. I

The present invention is therefore directed to I the combining of thepush-pull class A and class B type of systems to eliminate thedifilculty oi slit orientation and adjustment and to provide inherentnoise reduction.

Combination push-pull class A and class B systems are known in the art,one such system be- 7 ing disclosed and claimed in copending applicaofsaid beams, it is equallyincreased in the other,

light always reaching the film from each beam during the entireamplitude range-oi the signal. In the variable area type oi! system theposition of the tapered light beams at times of no signal is midwaybetween the vibration limits of the beams, and considerable light istherefore impressed upon the film. This condition requires theintroduction of noise reduction to class A recording in order to producea quiet film during times of no signal, noise reduction generallyconsisting in the blocking oi! of an amount of light dependent upon theamplitude of modulation oi the light beam. However, the orientation orazimuth adjustment '0! the slit with respect to the light beams and thefilm is not critical with this arrangement, nor is any substantialdifiiculty encountered with process distortion since the light from eachbeam is on the, film at all times.

Referring now to the class B type oirecording,

. which has the great advantage of having inherent noise reduction, thissystem employs two, light beams similar to the push-pull class A system,exceptthat at times of no signal, substantially no light reaches thefilm. In variable area class B recording, tapered light beams arepositioned at times of no signal on each side of the tion of C. H.Cartwright, Serial No. 168,565, filed October 12, 1937. This disclosurepoints out that a certain definite relationship must exist when goingfrom the class A type of recording to the combination type, thisfundamental and basic relationship being that each beam must be madetwice as effective when the other beam is eliminated; or, in otherwords, that the light transmission reaching the cell must double ineffectiveness at the point of changeover. This fundamental relationshipis, of course, retained in the present invention, while securing all theboneficial features of push-pull class A recording and of class Brecording.

The principal object of this invention, there-- fore, is to facilitatethe recording of sound on film.

Another object of the invention is to combine push-pull class A andclass B recording systems i to eliminate the disadvantages of each andto secure the essential advantages of both.

A Iurther object of the invention is to produce a sound track, the lowlevels of which are recorded as class A, and the upper amplitude levelsof which arev recorded as a combination of class A and class B. j

A further object of the invention is to produce a soundtrack, the lowlevels of which are recorded as pure class A at one value of lightintensity,

It'is and the upper amplitude levels of which .are recorded as acombination of class A and class B at a difierent light intensity or asa combination 01' a plurality of light intensities.

A further object of the invention is to produce a combination class Aand class B sound track having both variable area and variable densitycomponents.

Although the novel features which are believed to be characteristic ofthis invention are pointed out with particularity in the claims appendedherewith, the manner of its organization and the mode of its operationwill be better understood by referring to the following description readin conjunction with the accompanying drawing forming a part thereof, inwhich Figure 1 is a diagrammatic view of a sound recording systemembodying the invention;

Figure 2 is an enlarged detailed view of the slit Referring now to Fig.1, a light source illustrated by a filament 5 produces light which iscollected by a lens 6 and projected on an aperture mask 1 havingapertures 8 and 9 therein. The light passing the apertures is projectedby a lens ll upon a mirror of a galvanometer I2. light is then reflectedto a slit mask it having a slit l5 therein, the light'beams passing-theapertures 8 and 9 being shown at l! and ill on the mask. The light fromthe beams passing the slit I5 is projected by lenses l9 upon the soundtrack portion 2| of a film 22 advanced in any well known manner.

In the above mentioned copending application,

'the shape of the apertures 8 and 9 was such that the width of the lightbeams reaching the film increased twice as rapidly for amplitudes of thesignal above a certain level, which level represented the cross-overbetween pure class A recording and combination class A and class B. Thepresent invention permits the'use of normal triangular-shaped aperturesnormally used in push-pull class A wherein they are overlapped so thatthe apex of one triangle lies in the same horizontal plane as the baseof the other triangle. In class B systems the same shaped equilateraltriangles are employed, except that the apices of each lie in the samehorizontal plane, and the apices of the light beams lie at the center ofthe slit. In the present invention the equilateral apertures areoverlapped, the amount of overlapping representing the proportion ofclass A track signal. However, it will be noted that a certain portionof eachtriangle adjacent the apioes is v shaded, this shadingrepresenting a lesser light intensity at these portions caused by theintroduction, in the apertures 8' and 9 of mask I, 01' neutral lightfilters II and I3. The position The of the light beams l1 and I8 on maskll is shown for a no signal condition, and it will be noted that thelight passing through the film is represented by the width of the beamimpressed on the slit 15. As the light beams vibrate normally to theslit and an amplitude of vibration is reached which is greater than thedistance between the slit to the edge of the filter shadows and 26, thefull intensity of the light beams will be impressed upon the film 22.-In other words, when one of the shadows 25 or 26 is eliminated fromreaching the film except for the tail sections 28 and 29, the lightintensity of the other beam reaching the film increases.

To conform to the fundamental relationship required to produce a directproportionality between input and reproduced output, it is necessarythat the light intensity be increased in an amount such that the finalprint will produce a decrease in light reaching the reproducingphotocell. Thus, the processing of the negative and positive shouldprovide the print with twice the light transmission in the class Aregion of the track than for the pure class B region. Thus, the densityof the class A modulations of the final print will not approachopaqueness, but will lie substantially midway between transmission andzero transmission. A certain amount of noise reduction will, of course,be represented by this opacity of the film at times of no signal butthis opacity is insumcient for the amount of noise reduction desired inmodern sound track recording. Furthermore, a gray opacity introducesnoise because of its graininess, thus producing a certain hiss. Thus, itis preferable to provide the trace of the class A region with a greateropacity. Of course, the use offine grained film will reduce the grainnoise to a minimum.

Referring now to Fig. 3, a variation in the apertures has been shownwherein the class sections 3! and 32 of the light beams are shownconsiderably darker to represent lower light transmission filters. Thus,for the class A portion of the recording, a very high degree of noisereduction is introduced, this noise reduction being that of the orderobtained in normal variable density recording systems. However, it isnecessary, in the modification shown in Fig. 3, to retain thefundamental relationship at the points of cross-over between pure classA and combination A and B. To obtain this relationship, there have beeninserted in the remaining portions of the apertures 8 and 9 gray filtershaving a transmission similar to those used in Fig. 2, asshown by thegray regions 33 and 34. However, this carrier film density introducesundesired ground noise.

To overcome all difliculties, however, the modification of the aperturemask shown in Fig, 4 is employed, this figure showing the light mask l4and slit IS with the light beams shown thereon at -times of no signal.Itwill be noted that the light beams are identical in symmetry and inconstruction and have the same relative position as those of Figs. 2 and3. In this modification, the filters used in the pure class A sectionrepresented in the beams at 35 and 36 have the density of those of- Fig.3 to produce a high degree of noise reduction in the pure class Aregion. However, the remaining portions of the light beams are at fullintensity as shown at 3'! and 38, these portions of the aperture nothaving any filters. This change in light intensity between the beamportions 35 and 36 and the beam portions 31 and the pure class A portionof the traces.

. 2,284,781 38 does not conform to the fundamental relaadvantages ofclass A and the advantages of class.

B are obtained.

To further explain the operation of this type of recording system,reference :is made to Fig. 5, wherein galvanometer deflection is plottedagainst light transmission through the final sound record. With the typeof filter shown in Figs. 3 and 4 for the classA region, the lightreaching the film at times of no signal maybe represented i at slightlyabove zero light transmission. Then,

over. the class A portion, a linear relationship ex-,

, ists between galvanometer deflection and light transmission, sincethese portions of the. apertures have straight lines. However, at thepoint of cross-over, as represented by the point e, the

light transmission will increase tothe point I, at

which point the light transmission will vary along a straight line forfurther galvanometer' deflection. The class A section is represented bythe distance x, the remaining abscissa being combination class A andclass B. To conform to the fundamental cross-over relationship, the

distance 'a must be equal to the distance I). If a larger class Aportion is desired, such as repreborhood of 5 or 6 used for variablearea films. Thus, the filters will produce the proper density on thefinal print with the normal variable processing. It is to be understoodthat other shapes of apertures may be so related to the filter densitiesas to produce other variations such as compression or expansion.Furthermore, the separation between sections and 36 and sections 31 and38 may he graduated in density, the change in dimensions of the beamsbeing similarly graduated by curving the sides of the apertures at thepoint where they change in slope.

- What I claim as my invention is:

1. A sound recording system comprising means for obtaining a, pluralityof light beams, and

means for varying the light values of said beams in accordance with asignal, said light beams having a certain intensity and varying indimensions as a class A recording for amplitudes of saidsignal within a.certain amplitude range of said signal, and varying similarly indimensions as a combination class A and class B recording for anotheramplitude range of said signal, said. beams having a different intensitywhen varying as a combination class A and class B recording, thevariations in said light beams corresponding to the amplitudes of saidsignal at all amplitudes thereof.

2. A sound recording system comprisin a pinrality of light beams, meansfor vibrating said beam in accordance with a'signal to be recorded,

' and a film upon which said light beams are impressed, said light.beams having sizes, shapes and one variation in intensity to produce aclass A record'on said film, and other sizes, shapes and anothervariation in intensity to produce a comsented by the distancey, it maybe obtained by making the distance 0 and the distance d equal to oneanother, and a light transmission curve, such as shown bythedQt-and-dash line, is ob tained. Another curve is shown by the dottedline showing a case where the light transmission is higher for the classA portions, such as might be obtained with filters giving an intensityas shown at 25 and 26 in Fig. 2. 1

The form of negative track producible with this type of combinationclass A and class B system is shown in Fig. 6, wherein the gray portions40 and ll represent the no signal condition, while the modulations 42and 43 represent the pure class A section, which, of course, will havethe same density as the sections 40 and II. For higher levelsofcross-over, the wider the traces shown at 40 and 4|. At higheramplitudes represented by themodulations H and 45, two densitycomponents are found, the higher amplitudes being, completely opaque,while the intermediate sections have a density corresponding to that ofThe print of this negative of course, will have reversed densities, theentire unmodulated areas being completely opaque, the areas 40 and 43having a very low light transmission, and the peak areas oi modulationsM and 45 havingsubstantially bination class A and class B sound recordon said film, said variations being proportional to the respectiveamplitudes of said signal.

3. In combination, a source of light, means for accordance with asignal, and afilm upon whichthe emergent light beams from said slit areprojected, said light beams having sizes, shapes and variations inintensity such that for amplitudes, of vibration up to a predeterminedpoint, said emergent beams produce a push-pull class A track of onedensity upon said film, and for amplitudes of vibration above .saidpredetermined point, said emergent light beams produce a combinationclass A- class B track having a plurality of densities upon said film,the variations in said light beams corresponding in a linear manner toall the amplitudes of said signal. I

4. In a sound recording system, means for producing a plurality'of lightbeams, means for projecting said beams uponan elongated slit, a filmhaving a motion transversely of said slit, and means for vibrating saidlight beams in the direction of movement of said film, said light beamshaving a shape and variation in intensity and so positioned with respectto said slit that parallel beams of one intensity are projected sibeingproportional at all times to a signal being recorded.

5. In a sound recording system, a sourceof light, a mask for producingspatially disposed light beams, and a mask having an elongated slittherein upon which said beams are projected, said first mentioned maskhaving spatially disabove acertain level as combination class A andclass B, all 01' said amplitudes corresponding to g the respectiveamplitude of a recorded signal.

posed apertures therein with overlapping portions with respect todifferent portions of said slit, said overlapping portions containing afilter for decreasing the light intensity of certain respective portionsof said beams.

6. A light defining mask having a pair of substantiallytriangular-shaped apertures therein horizontally disposed with respectto one another, and oppositely opposed vertically, the width of saidapertures in a vertical direction increasing in a predeterminedproportion over a certain length and varying in a different proportionover the remainder of their length, one portion of each aperturecontaining a neutral light filter.

7. A light defining mask in accordance with claim 6 in which the changein width of the remaining length of said apertures varies in accordancewith the density value of said filters.

8. A film sound track having two substantially parallel continuoustraces of a certain density extending longitudinally of the film forsignal amplitudes below a certain predetermined amplitude level, andhaving two substantially parallel discontinuous traces of difierentdensities for amplitudes above said certain predetermined level.

9. A push-pull film sound track having traces, each trace containingcontinuous modulations of one density for amplitudes below apredetermined level, and each trace containing discontinuous modulationsof a plurality of densities for amplitudes above said level, all of saidmodtudes above said point, all or said modulations corresponding to therespective amplitudes of the recorded signal.

11. In a sound recording system, the method of recording a sound trackwhich comprises recording at a definite lightintensity amplitudes belowa certain level as class A, and recording at a plurality of lightintensities amplitudes 12. In a sound recording system, the method ofproducing a combination class A and class B sound record comprisingrecording at one light intensity signals below a certain predeterminedlevel as pure class A signals, and recording at said one light intensityand another light intensity all amplitudes of said signals above saidcertainlevel as combination class A and class 3, all recorded amplitudescorresponding in a linear manner to all amplitudes of said signals.

13. In a sound recording system, the method of recording a sound trackwhich comprises recording amplitudes within a certain range at one lightintensity, and amplitudes within another range as a combination of saidone light intensity and another light intensity, all of saidamplitudes'corresponding to the respective amplitudes of a recordedsignal.

14. In a push-pull sound recording system, the

. method of recording a combination class A and class B sound trackcomprising simultaneously varying the lengths of spatially disposedlight beams such that the decrease in length of one beam is proportionalto the increase in length of the other of said beams for signals up to apredetermined amplitude, said beams having a certain predetermined lightintensity, and for signals above said predetermined amplitude, saidlight beams having a diiferent intensity, the variations in the lengthof each of said beams being at a rate in accordance with the differencein the intensities 01' said beams.

15. A push-pull variable area sound recording system in accordance withclaim 14 in which the variations in the length of said light beams for asignal having amplitudes above said predetermined amplitude occurs atmutually exclusive time intervals.

16. A sound track having a plurality of traces,

at least one of which has a constant density and contains the completevariations of a recorded signal for amplitudes below a predeterminedamplitude value, said trace havinga plurality of densities andcontaining only a portion of the variations of said recorded signal foramplitudes above said predetermined amplitude value, the remainingportion of said variations of said recorded signal within the amplituderange above said predetermined amplitude value being contained in saidother trace or traces and having a plurality of densities.

' IRL R. GOSHAW.

